[cfe-dev] Clang function-sections flag

[Reid Kleckner via cfe-dev]

Looks like everything is set, but let me try to explain a little bit.

The /O1 and /O2 options of MSVC and clang-cl actually expand to several
other options. You can see this in the docs:
https://docs.microsoft.com/en-us/cpp/build/reference/o1-o2-minimize-size-maximize-speed?view=msvc-160
/O2 (Maximize Speed) /Og /Oi /Ot /Oy /Ob2 /GF /Gy

In particular, /Gy means -ffunction-sections. If you were building your
code with optimizations enabled, (`clang-cl /O2 /c foo.cpp`), that would
explain why you see one section per function.

If you want to disable function sections more directly, consider passing
the /Gy- flag after the /O2 flag.

On Wed, Jan 20, 2021 at 7:17 AM Gaier, Bjoern via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Allright :’D
>
>
>
> That worked! Now my 100x functions are in one section! Thank you so much
> guys!
>
>
>
> *From:* Keane, Erich <erich.keane at intel.com>
> *Sent:* 20 January 2021 16:13
> *To:* Gaier, Bjoern <Bjoern.Gaier at horiba.com>; paul.robinson at sony.com;
> clang developer list <cfe-dev at lists.llvm.org>
> *Subject:* RE: Clang function-sections flag
>
>
>
> /clang: sends to the ‘driver’.  -Xclang sends to the ‘compiler instance’
> (the -cc1 compiler), which the driver calls.  They are actually the same
> executable which makes it a touch more confusing.
>
>
>
> *From:* Gaier, Bjoern <Bjoern.Gaier at horiba.com>
> *Sent:* Wednesday, January 20, 2021 7:11 AM
> *To:* Keane, Erich <erich.keane at intel.com>; paul.robinson at sony.com; clang
> developer list <cfe-dev at lists.llvm.org>
> *Subject:* RE: Clang function-sections flag
>
>
>
> Using /clang:-fno-function-sections and /clang:-ffunction-sections were
> both accepted by Clang-cl while /clang:-planschbecken was rejected.
>
>
>
> Seems like that worked O_O Thank you!
> I gonna check if this will make a difference now.
>
>
>
> In the meantime… why isn’t /clang: and -Xclang the same?!
>
>
>
> *From:* Keane, Erich <erich.keane at intel.com>
> *Sent:* 20 January 2021 16:07
> *To:* Gaier, Bjoern <Bjoern.Gaier at horiba.com>; paul.robinson at sony.com;
> clang developer list <cfe-dev at lists.llvm.org>
> *Subject:* RE: Clang function-sections flag
>
>
>
> Looks like CloudABI is the only triple that uses it by default. Presumably
> you aren’t on that, but I could also be missing something.
>
>
>
> For me, it looks like BOTH -ffunction-sections and -fno-function-sections
> are unknown arguments in clang-cl, however if you use the /clang: syntax,
> both work.  Can you try that?
>
>
>
> /clang:-fno-function-sections
>
>
>
> *From:* cfe-dev <cfe-dev-bounces at lists.llvm.org> *On Behalf Of *Gaier,
> Bjoern via cfe-dev
> *Sent:* Wednesday, January 20, 2021 7:00 AM
> *To:* paul.robinson at sony.com; cfe-dev at lists.llvm.org
> *Subject:* Re: [cfe-dev] Clang function-sections flag
>
>
>
> Hey Paul,
>
>
>
> I tried that already and got:
>
> “error: unknown argument: '-fno-function-sections'”
>
>
>
> I use Clang-Cl under Windows, creating an ELF file for
> x86_64-pc-windows-elf – however the flag is not supported (not with -Xclang
> but also not without)
>
> However “-ffunction-sections” is accepted (with -Xclang)
>
>
>
> Kind greetings
>
> Björn
>
>
>
> *From:* paul.robinson at sony.com <paul.robinson at sony.com>
> *Sent:* 20 January 2021 15:56
> *To:* Gaier, Bjoern <Bjoern.Gaier at horiba.com>; cfe-dev at lists.llvm.org
> *Subject:* RE: Clang function-sections flag
>
>
>
> Try adding -fno-function-sections to make sure it’s turned off (sounds
> like it is on by default for your target).
>
> --paulr
>
>
>
> *From:* cfe-dev <cfe-dev-bounces at lists.llvm.org> *On Behalf Of *Gaier,
> Bjoern via cfe-dev
> *Sent:* Wednesday, January 20, 2021 5:01 AM
> *To:* Clang Dev <cfe-dev at lists.llvm.org>
> *Subject:* [cfe-dev] Clang function-sections flag
>
>
>
> Hey Clang-people,
>
>
>
> I have a nooby question about the -ffunction-sections flag of Clang.
>
> As far as I used to understand, setting this flag means, that every
> function gets their own section. Makes sense so far.
>
>
>
> Recently I started to look into the ELF format a bit so that subject came
> up. I created a macro to quickly generate me some functions:
>
> #define wuff(a, b) \
>
> a##b
>
>
>
> #define crap(num) \
>
> int wuff(schwimmflugel, num) () \
>
> { \
>
>     return num; \
>
> }
>
>
>
> Creating the object file and looking into it, I found a section header for
> every of my functions no matter if I set the -ffunction-sections flag or
> not.
>
> Also setting my function to static didn't changed a thing.... well besides
> from having those functions striped from my object file xD
>
>
>
> So... what does that flag effect?
>
>
>
> Kind greetings
>
> Björn
>
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> Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816,
> USt.ID-Nr. DE 114 165 789 Geschäftsführer: Dr. Hiroshi Nakamura, Dr. Robert
> Plank, Markus Bode, Takashi Nagano, Junichi Tajika, Ergin Cansiz.
>
> Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816,
> USt.ID-Nr. DE 114 165 789 Geschäftsführer: Dr. Hiroshi Nakamura, Dr. Robert
> Plank, Markus Bode, Takashi Nagano, Junichi Tajika, Ergin Cansiz.
> Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816,
> USt.ID-Nr. DE 114 165 789 Geschäftsführer: Dr. Hiroshi Nakamura, Dr. Robert
> Plank, Markus Bode, Takashi Nagano, Junichi Tajika, Ergin Cansiz.
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